This invention relates to a process for forming an insulating film used in a luminescent device such as a thin-film electroluminescent device which takes out a light through a transparent electroconductive film and a receptor which takes in a light through a transparent electroconductive film formed on a transparent substrate, and a process for producing the thin-film electroluminescent device.
Thin-film electroluminescent devices which emitt a light by applying an alternating electric field comprise an emission layer, a single or a plurality of insulating films formed on one side or both sides of the emission layer, and a pair of electrode layers at least one of which is transparent sandwiching the insulating films so as to emit electroluminescence with high brightness. The transparent electrode layer is usually formed on a transparent substrate such as a glass substrate. The thin-film electroluminescent device usually has a structure as shown in the attached drawing, wherein numeral 1 denotes a transparent substrate, numeral 2 denotes a transparent electrode, numeral 3 denotes a first insulating layer, numeral 4 denotes an emission layer, numeral 5 denotes a second insulating layer which is formed if necessary, and numeral 6 denotes a backside electrode.
As the first insulating layer, there is used at least one thin film of silicon nitride, silicon oxide and silicon oxynitride. Such a layer can be formed on a transparent electroconductive film formed on a transparent substrate such as a glass substrate by a vacuum deposition method, e.g. a resistance heating vacuum deposition method, an electron beam heating vacuum deposition, a sputtering method, or a plasma chemical vapor deposition (CVD) method.
According to the vacuum deposition method, the film produced has many pin holes and low resistance portions due to change in the composition, and in the case of applying an electric field between the transparent electrode (or electroconductive film) and the backside electrode (or electroconductive film) formed on the backside of the insulating film, a leak current passing through the insulating film increases, and at last dielectric breakdown takes place in the above-mentioned defect portions when the electric field is enlarged. Further, step coverage is worsened when projections or fine dusts are present in the transparent electroconductive film, or at side faces of the transparent electroconductive film on which a pattern is formed. As a result, there arise problems in that the above-mentioned insulating film is thinned at these portions, and the change in composition takes place so as to lower the electric resistance, which results in increasing the leak current and causing dielectric breakdown.
According to the sputtering method, the step coverage is better than the vacuum deposition method but still insufficient. Further, there are many problems in that the film forming time is long, the productivity is worse, and the transparent electroconductive film is damaged when exposed to high energy plasma to increase the electric resistance.
According to the plasma CVD method, the problems in the sputtering method can almost be solved, but there arise other problems in that cracks are formed in the insulating film or the insulating film is peeled off in a heating step after the film formation of the insulating film. This phenomenon takes place remarkably, when the heating temperature in the heating step after the film formation is higher or the film formation area is larger. According to the process disclosed in Japanese Patent Unexamined Publication No. 61-18889, two Si-N films are formed on an emission layer by a plasma CVD method using a gas mixture of SiH.sub.4 and N.sub.2, followed by using a different gas mixture of SiH.sub.4 and NH.sub.3, at the substrate temperature of 100 to 300.degree. C. Since the emission layer has been formed after the formation of SiO.sub.2 film and Si-N film by sputtering on a glass substrate having a transparent electrode thereon, there is no problem of causing cracks on the overlying Si-N films during the heating step after the film formation, but the plasma CVD method employed therein is too complicated and lacks in productivity. In addition, according to the plasma CVD method, the resulting insulating film sometimes becomes opaque due to non-uniform film wherein the leak electric current becomes abnormally large.